Semiconductor devices and liquid crystal display devices are manufactured through a technique known as photolithography technique, by which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in the photolithography process has a mask stage that supports a mask and a substrate stage that supports a substrate, and while successively moving the mask stage and the substrate stage, transfers the mask pattern via a projection optical system onto the substrate. In recent years, there has been demand for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has also increased. Furthermore, the current mainstream exposure wavelength is 248 nm (KrF excimer laser), but an even shorter wavelength of 193 nm (ArF excimer laser) is now gradually being put to practical use. In addition, as well as resolution, the depth of focus (DOF) is also important when performing an exposure. The resolution R and the depth of focus δ are respectively expressed by the following formulas:R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)where λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. It can be seen from formulas (1) and (2) that if, to enhance the resolution R, the wavelength λ is shortened and the numerical aperture NA is increased, then the depth of focus δ becomes narrower.
When the depth of focus δ becomes too narrow, it becomes difficult to make the substrate surface coincide with the image plane of the projection optical system, and thus there is the possibility that the focus margin during the exposure operation will be insufficient. To address this problem, the liquid immersion method, which is disclosed in, e.g., Patent Document 1 below, has been proposed as a method to make the exposure wavelength substantially shorter and to make the depth of focus substantially broader. This liquid immersion method is designed to fill the space between the under surface of the projection optical system and the substrate surface with a liquid, e.g., water or organic solvent, to form a liquid immersion region and thus by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times (n is the refractive index of the liquid and is generally approximately 1.2 to 1.6) of that in the air, improve the resolution and, at the same time, enlarge the depth of focus by approximately n times.
Patent Document 1: PCT International Publication No. WO 99/49504 pamphlet